Wisdom 18:12

It commemorates what the Book of Exodus recounts about the killing of the firstborn, when the Lord struck in the middle of the night every firstborn in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the captive woman who was in the prison, and every firstborn of the animals (Ex. 11). There, unlike before when they pretended not to agree to the departure of the Israelites, they openly urged the people of God to leave the land of Egypt quickly, saying: We shall all die.

Similarly therefore all, that is, the greater and the lesser and the middling, namely King Pharaoh and the other lords and their servants: had innumerable dead, all the firstborn, dead, I say, by one manner of death, namely by one kind, because it was a divine plague. And he well says: all, because "there was no house in which there did not lie one dead," as is found in Exodus twelve. For neither etc. And I well said: innumerable: for neither for burying, namely the dead, did the living suffice: Jeremiah sixteen: "The small and the great shall die, and they shall not be buried" etc. Since in one moment, namely in one hour of the night, that which was the more noble offspring of theirs, that is, the whole body of the firstborn, who surpass the remaining sons in dignity: was destroyed, that is, slain by the destroying Angel, as is clear from Exodus twelve.